feat(sync-v2): Improve overall logging

[msbrogli]

Motivation

After running our latest QA, we noticed that the sync-v2 logging can be improved in a way that we don't get too many duplicated log messages but got enough messages to debug an issue.

Acceptance Criteria

1. Add _HeightInfo.__str__() because it is used by the log json serializer.
2. Only log "blocks are synced" and "nothing to sync because peer is behind me at the same best blockchain" when we transition from not synced to synced. So it stops logging duplicated messages all the time.
3. Improve other log levels and messages.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (d0a1ff2) 85.50% compared to head (db38db4) 85.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #854      +/-   ##
==========================================
- Coverage   85.50%   85.33%   -0.18%     
==========================================
  Files         282      282              
  Lines       22242    22247       +5     
  Branches     3361     3363       +2     
==========================================
- Hits        19018    18984      -34     
- Misses       2562     2595      +33     
- Partials      662      668       +6     

Files	Coverage Δ
hathor/p2p/sync_v2/blockchain_streaming_client.py	81.01% <100.00%> (ø)
hathor/p2p/sync_v2/streamers.py	90.86% <100.00%> (ø)
hathor/p2p/sync_v2/agent.py	78.21% <90.90%> (-2.07%)	⬇️
hathor/p2p/sync_v2/transaction_streaming_client.py	79.20% <50.00%> (-10.90%)	⬇️

... and 4 files with indirect coverage changes

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